UCC28633: Improving cross-rail regulation and optimising the overall design

Hello Francesco,

Thank you for your interest in the UCC28633 Flyback controller.

Transformer construction does have an influence in multi-output cross-regulation, mainly based on the leakage inductance distribution amongst all of the windings.  But additional "leakage" on each winding may be added by loop area of the pcb tracks depending on the path length and width through the output diode(s) and the filter caps back to the winding. 

During the flyback interval the energy stored in the core magnetic field will discharge amongst the windings preferentially through the paths with the lowest inductance first.  These paths charge up their capacitances until the voltages are high enough to block further discharge and the remaining energy then goes into the next lowest path, and so on.  Outputs with no loads are susceptible to overvoltage as you are experiencing.

Leakage inductances are unavoidable, but may be managed to try to balance the energy flow more evenly. This may be a trial and error process, mitigated somewhat by analytical reasoning about which pcb path may be increased or decreased to drive the stray inductance in a particular direction.  Also, the order of the transformer winding structure can be rearranged.  I'm sorry that I can't offer more than these generalities.  It is an iterative process, unfortunately, unless you have extensive finite element analysis capability.

But I do recommend adding some minimum loading to each output until you get the voltages under control, then backing off on the preloads as things improve.

To reduce the heat in your primary clamp, I recommend to go exclusively with TVS's and get rid of the RCD.  Also, increase each TVS to SMC size, instead of SMB, for better heat dissipation.  Finally, increase the voltage of each TVS to the highest you can tolerate given your maximum input voltage, reflected voltage and least tolerable MOSFET derating.  A higher clamp voltage will tend to lower the overall clamp losses closer to the theoretical leakage energy times frequency.  A second benefit of all TVS is that standby power should be reduced.  This is because at light load or no load, the RCD clamp discharges significantly between pulses and more primary energy is needed to recharge the clamp before any energy can be delivered to the output. The TVSs do not need recharging, so standby frequency can be lower and consequently so is standby power. This can mitigate some of the preloading on the outputs to some extent.

One drawback of TVS clamps is the sudden on and off of the clamp can aggravate EMI more than the softer RCD clamp. Ther are ways to mitigate this too, but your first priority is to get your system running.

Your last question about the 250uH inductance in PMP30092 is a good one.  I don't know for sure myself, but I suspect that a 160W DCM flyback would have excessively high peak currents, so the designer chose higher Lm to force most of the operation into CCM to bring the peak current down.  I can't think of any other good reason.

Good luck with your cross-regulation efforts, and don't forget to involve the pcb layout.  Possibly reduce some loop areas and maybe deliberately increase a few select others. 

Regards,

Ulrich